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Unit 1:
Ch. 1
Ch. 2
Ch. 3
Interlude A
Unit 2:
Ch. 4
Ch. 5
Ch. 6
Ch. 7
Ch. 8
Ch. 9
Interlude B
Unit 3:
Ch. 10
Ch. 11
Ch. 12
Ch. 13
Ch. 14
Ch. 15
Interlude C
Unit 4:
Ch. 16
Ch. 17
Ch. 18
Ch. 19
Interlude D
Unit 5:
Ch. 20
Ch. 21
Ch. 22
Ch. 23
Ch. 24
Ch. 25
Ch. 26
Ch. 27
Ch. 28
Ch. 29
Ch. 30
Interlude E
Unit 6:
Ch. 31
Ch. 32
Interlude F
Unit 7:
Ch. 33
Ch. 34
Ch. 35
Ch. 36
Ch. 37
Ch. 38
Interlude G

» Getting Started » A Guide to the Reading » Tying it all together

Getting Started

Below are a few questions to consider prior to reading Chapter 9. These questions will help guide your exploration and assist you in identifying some of the key concepts presented in this chapter.

  1. How does the microorganism Borrelia, the causative agent of Lyme disease, cause the body’s immune system to undergo an identity crisis?
  2. What are the differences between the G1 and G2 phases of the cell cycle?
  3. What role do the cytoskeletal structures called microtubules play in cell division?
  4. What important role do vesicles play in the process of plant cell division?
  5. What is the primary difference between haploid and diploid cells?
  6. How many copies of each chromosome are present in cells that have undergone meiosis I?  How about meiosis II?

A Guide to the Reading

When exploring the content in Chapter 9 for the first time, the following concepts typically give students the most difficulty. For each concept, one or more references have been identified which may help you gain a better understanding of these potentially problematic areas.

Homologous Chromosomes

As described in the chapter, homologous chromosomes are pairs of chromosomes containing the same set of genes.  One homolog of the pair is inherited from the organism’s mother, while the other homolog is inherited from the father.  Humans possess 23 pairs of homologous chromosomes for a grand total of 46 individual chromosomes.  The process of cell division must utilize a mechanism which ensures that each daughter cell obtains a complete set of 23 pairs of homologous chromosomes.  To accomplish this, each individual chromosome must be copied.  This occurs during the S phase of the cell cycle.  After a chromosome is copied, the two duplicate strands of DNA (which are physically joined together by a structure located in the center called a centromere) are referred to as chromatids.  With each of the 46 chromosomes having an attached duplicate, the cell can then begin the process of cell division.  By aligning the chromatids in the center of the cell during “metaphase”, the cell ensures that each side of the cell will obtain one copy of each of the 46 chromosomes following cell division.

For more information on this concept, be sure to focus on:

  • In Section 9.3, The DNA of each species is organized into a distinctive karyotype
  • Figure 9.3, The Packing of DNA into a Chromosome
  • Figure 9.4, Human Chromosomes
  • Figure 9.5, The Stages of Cell Division


The key to understanding meiosis is knowing the overall goal of the process – to reduce the number of chromosomes in the daughter cells (gametes).  Gametes in sexually reproducing species are haploid’ that is, they contain only one copy of each chromosome.  This process of producing haploid cells requires two cell divisions occurring in sequence, called meiosis I and meiosis II.  The key step in the process is the pairing of homologous chromosomes, forming bivalents, during meiosis I.  This pairing process does not occur during mitosis.  When these bivalents are split during anaphase I, one complete chromosome (consisting of two identical chromatids) is segregated into each of the two daughter cells.  The result is that each daughter cell formed after meiosis I has only one complete set of chromosomes, and therefore would be considered haploid.  The process of meiosis II then serves to separate the chromatids into one of the two resulting daughter cells.  In the end, the process of meiosis produces four gametes, each with one copy of each of the chromosomes.

For more information on this concept, be sure to focus on:

  • In Section 9.4, Meiosis I is the reduction division
  • In Section 9.4, Meiosis II is similar to mitosis
  • Figure 9.8, Similarities and Differences between Meiosis and Mitosis

Tying it all together

Several concepts presented in this chapter build upon concepts presented in previous chapters and are also revisited and discussed in greater detail in subsequent chapters, including:

Cell Communication

  • Chapter 6 –Section 6.6, Signaling Molecules in Cell Communication

Chromosomes and Genetics

  • Chapter 11 –Section 11.1, The Role of Chromosomes in Inheritance

Crossing Over

  • Chapter 11 – Section 11.3, Linkage and Crossing-Over

Human Reproduction

  • Chapter 29, Reproduction and Development

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